Defining North Poles in astronomy

Astronomers define the north "geographic" pole of a planet or other object in the solar system by the planetary pole that is in the same ecliptic hemisphere as the Earth's north pole. More accurately, «The north pole is that pole of rotation that lies on the north side of the invariable plane of the solar system» [1] (http://www.hnsky.org/iau-iag.htm). This means some objects will have directions of rotation opposite the "normal" (i.e., not counter-clockwise as seen from above the north pole). Another frequently used definition uses the right-hand rule to define the north pole: it is then the pole around which the object rotates counterclockwise [2] (http://nssdc.gsfc.nasa.gov/planetary/factsheet/index.html). When using the first definition (the IAU's), an object's axial tilt will always be 90° or less, but its rotation period may be negative (retrograde rotation); when using the second definition, axial tilts may be greater than 90° but rotation periods will always be positive.

For the magnetic poles, their names are decided upon by the direction that their field lines emerge or enter the planet's crust. If they enter the same way as they do for Earth at the north pole, we call this the planet's north magnetic pole.

In the particular (but frequent) case of synchronous satellites, four more poles can be defined. They are the near, far, leading, and trailing poles. Take Io for example; this moon of Jupiter rotates synchronously, so its orientation with respect to Jupiter stays constant. There will be a single, unmoving point of its surface where Jupiter is at the zenith, exactly overhead —this is the near pole, also called the sub- or pro-Jovian point. At the antipode of this point is the far pole, where Jupiter lies at the nadir. There will also be a single unmoving point which is furthest along Io's orbit (best defined as the point most removed from the plane formed by the north-south and near-far axes, on the leading side) —this is the leading pole. At its antipode lies the trailing pole. Io can thus be divided into north and south hemispheres, into pro- and anti-Jovian hemispheres, and into leading and trailing hemispheres. Note that these poles are mean poles because the points are not, strictly speaking, unmoving: there is constant jiggling about the mean orientation, because Io's orbit is slightly eccentric and the gravity of the other moons disturbs it regularly.

Defining the North Pole of Earth

The North Pole, the northernmost point on the Earth, can be defined in four different ways. Only the first two definitions are commonly used. However it is defined, the North Pole lies in the Arctic Ocean.

The Geographic North Pole, also known as True North, is approximately the northern point at which the Earth's axis of rotation meets the surface. (see next section)

The Magnetic North Pole is the northern point at which the geomagnetic field points vertically, i.e. the dip is 90°.

The Geomagnetic North Pole is the northern pole of the Earth's geomagnetic field's dipole moment.

Geographic North Pole

The Geographic North Pole, also known as True North, is close to the northern point at which the Earth's axis of rotation meets the surface. Geographic North defines latitude 90° North. In whichever direction you travel from here, you are always heading south. The pole is located in the Arctic Ocean, which at this point has a depth of 4087 metres (13,410 feet). Classically (19th century) this pole was exactly where people believed the pole of rotation met the Earth's surface, but soon astronomers noticed a small apparent variation of latitude as determined for a fixed point on Earth by observing stars. This variation had a period of about 435 days and the periodic part of it is now called the Chandler wobble after its discoverer. It is desirable to tie the system of Earth coordinates (latitude, longitude, and elevations or orography) to fixed landforms. Of course, given continental drift and the rising and falling of land due to volcanos, erosion and so on, there is no system in which all geographic features are fixed. Yet the International Earth Rotation and Reference Systems Service and the International Astronomical Union have defined a framework called the International Terrestrial Reference System that does an admirable job. The North pole of this system now defines geographic North and it does not quite coincide with the rotation axis. Also see polar motion.

On the basis of the sector principle, Canada claims its sovereignty to extend all the way to the Geographic North Pole. There is no land at this location, which is usually covered by sea ice. The theory under which Canada has claimed sovereignty to the North Pole is controversial as there are in fact 770 km of ocean between the pole and Canada's northernmost point, and several nations, most notably the United States, have challenged the notion that the North Pole does not lie in international waters.

The first expedition to the pole is generally accepted to have been made by Navy engineer Robert Edwin Peary and his employee, African-American Matthew Henson and four Inuit men (Ootah, Seegloo, Egingway, and Ooqueah) on April 6, 1909. Polar historians believe that Peary honestly thought he had reached the pole. However a 1996 analysis of a newly-discovered copy of Peary's record indicates that Peary must have been in fact 20 nautical miles (40 km) short of the Pole.

Magnetic North

Magnetic North is one of several locations on the Earth's surface known as the "North Pole". Its definition, as the point where the geomagnetic field points vertically downwards, i.e. the dip is 90°, was proposed in 1600 by Sir William Gilbert, a courtier of Queen Elizabeth I, and is still used. It should not be confused with the less frequently used Geomagnetic North Pole. Magnetic North is the place to which all magnetic compasses point, although since the pole marked "N" on a bar magnet points north, and only opposite magnetic poles are attracted to each other, the Earth's magnetic north is actually a south magnetic pole.

The orientation of magnetic fields of planets can flip over. The Earth's poles have done this repeatedly throughout history, and 500,000 years ago, the south magnetic pole was at the North Pole. It is thought that this occurs when the circulation of liquid nickel/iron in the Earth's outer core is disrupted and then reestablishes itself in the opposite direction. It is not known what causes these disruptions.

The first expedition to reach this pole was led by James Clark Ross, who found it at Cape Adelaide on the Boothia Peninsula on June 1, 1831. Roald Amundsen found Magnetic North in a slightly different location in 1903. The third observation of Magnetic North was by Canadian government scientists Paul Serson and Jack Clark, of the Dominion Astrophysical Observatory, who found the pole at Allen Lake on Prince of Wales Island. The Canadian government has made several measurements since, which show that Magnetic North is continually moving northwest. Its location (in 2003) is 78°18' North, 104° West, near Ellef Ringness Island, one of the Queen Elizabeth Islands, in Canada. During the 20th century it has moved 1100 km, and since 1970 its rate of motion has accelerated from 9 km/a to 41 km/a (2001-2003 average; see also Polar drift). If it maintains its present speed and direction it will reach Siberia in about 50 years, but it is expected to veer from its present course and slow down.

This movement is on top of a daily or diurnal variation in which Magnetic North describes a rough ellipse, with a maximum deviation of 80 km from its mean position. This effect is due to disturbances of the geomagnetic field by the sun. A line drawn from one magnetic pole to the other does not go through the centre of the Earth, it actually misses it by about 530 km.

The angular difference between Magnetic North and true North varies with location, and is called the magnetic declination.

Geomagnetic North Pole

The Geomagnetic North Pole is the pole of the Earth's geomagnetic field closest to true north. The first-order approximation of the Earth's magnetic field is that of a single magnetic dipole (like a bar magnet), tilted about 11° with respect to Earth's rotation axis and centered at the Earth's core. The residuals form the nondipole field. The Geomagnetic poles are the places where the axis of this dipole intersects the Earth's surface. Because the dipole approximation is far from a perfect fit to the Earth's magnetic field, the magnetic field is not quite vertical at the geomagnetic poles. The locations of true vertical field orientation are the magnetic poles, and these are about 30 degrees of longitude away from the geomagnetic poles.

Like the Magnetic North Pole, the geomagnetic north pole is a south magnetic pole, because it attracts the north pole of a bar magnet. It is the centre of the region in the magnetosphere in which the Aurora Borealis can be seen. Its present location is 78°30' North, 69° West, near Thule in Greenland. The first voyage to this pole was by David Hempleman-Adams in 1992.

The Northern Pole of Inaccessibility

The Northern Pole of Inaccessibility, located at 84°03' north, 174°51' west, is the point farthest from any northern coastline, about 1100 km from the nearest coast. It is a geographic construct, not an actual physical phenomenon. It was first reached by Sir Hubert Wilkins, who flew by aircraft in 1927; in 1958 a Russianicebreaker reached this point.

Territorial claims to the North Pole

Until 1999, the North Pole had been considered international territory. However, as the polar ice has begun to recede at a rate higher than expected (see global warming), several countries have made moves to claim the water or seabed at the Pole. Russia made its first claim in 2001, claiming Lomonosov Ridge, an underwater mountain ridge underneath the Pole, as a natural extension of Siberia. This claim was contested by Norway, Canada, the United States and Denmark in 2004. Denmark's territory of Greenland has the nearest coastline to the North Pole, and Denmark argues that the Lomonosov Ridge is in fact an extension of Greenland. The potential value of the North Pole and the area around resides in any possible potential oil and gas below the underlying sea-bed, the exploration for which in the near future might become more feasible after the opening of the Northwest Passage.